Magnetic resonance imaging (MRI) is a technique that is capable of providing three-dimensional imaging of an object. A conventional MRI system typically includes a main or primary magnet that provides the main static magnetic field Bo, magnetic field gradient coils and radio frequency (RF) coils, which are used for spatial encoding, exciting and detecting the nuclei for imaging. Typically, the main magnet is designed to provide a homogeneous magnetic field in an internal region within the main magnet, for example, in the air space of a large central bore of a solenoid or in the air gap between the magnetic pole plates of a C-type magnet. The patient or object to be imaged is positioned in the homogeneous field region located in such air space. The gradient field and the RF coils are typically located external to the patient or object to be imaged and inside the geometry of the main or primary magnet(s) surrounding the air space. There is shown in U.S. Pat. Nos. 4,689,563; 4,968,937 and 5,990,681, the teachings of which are incorporated herein by reference, some exemplary MRI systems.
In MRI, high-resolution information is obtained on liquids such as intracellular or extra-cellular fluid, tumors such as benign or malignant tumors, inflammatory tissues such as muscles and the like through the medium of a nuclear magnetic resonance (NMR) signal of a nuclear magnetic resonance substance such hydrogen, fluorine, magnesium, phosphorous, sodium, calcium or the like found in the area (e.g., organ, muscle, etc.) of interest. In addition to being a non-invasive technique, the MRI images contain chemical information in addition to the morphological information, which can provide physiological information.
Most clinical uses of MRI of biological tissue, however, employ the water content and water relaxation properties to image anatomy and function with micro-liter resolution. The relaxation properties of water (through 1H nuclei) are the basis for most of the contrast obtained by NMR imaging techniques. Conventional 1H NMR images of biological tissues usually reflect a combination of spin-lattice (T1) and spin-spin (T2) water 1H relaxation. The variations in water 1H relaxation rate generate image contrast between different tissue and pathologies depending upon how the NMR image is collected.
With MRI based on 1H water relaxation properties, the system typically detects signals from mobile protons (1H) that have sufficiently long T2 relaxation times so that spatial encoding gradients can be played out between excitation and acquisition before the signal has completely decayed. The T2-values of less mobile protons associated with immobile macromolecules and membranes in biological tissues are too short (e.g., less than 1 ms) to be detected directly in the MRI process.
In MRI, one of the most common problems or issues to solve relates to the selective imaging of a particular tissue type (i.e., T1 species) or body component or the like of interest, without contamination or too much influence from other tissue types or body components of a specific location of the body. There are several possible techniques for MR imaging to eliminate unwanted tissue or body components. One is using the difference in spatial location, such as in outer volume fat suppression in MR spectroscopic imaging. Another is using the difference in resonance frequency, mostly used for elimination of any residual fat signal in most MR imaging or spectroscopic techniques.
The technique that is least sensitive to local variations in magnetic field or moving spins (such as in blood or CSF) is to use the difference in longitudinal relaxation time (T1) between different tissues types. This longitudinal relaxation time (T1) is an inherent property of any body component and is different for any organ and it represents the time constant by which the magnetization exponentially return to its original equilibrium value after application of an RF excitation pulse. Below are three different examples of magnetic resonance sequences utilizing such nulling preparation schemes of a moving fluid.
First, a functional MRI (fMRI) technique was recently developed/introduced that can be used to sense cerebral blood volume (CBV) changes during neural activity referred to as Vascular-Space-Occupancy (VASO) that uses a non-selective inversion pulse with an optimal inversion time (TI) to eliminate blood signals. More precisely, an inversion-recovery sequence is used whereby an inversion pulse or sequence of pulses (resulting in a 180 deg. pulse) inverts the magnetization non-selectively in the entire brain, and an optimal time is provided in order for the blood magnetization to recover and cross the zero line. All other tissue parts present in the brain (CSF, gray matter, white matter) will have some remaining magnetization due to the difference in their T1 relaxation time with the T1 relaxation time of the blood. In VASO, the remaining tissue signal is thus determined by the amount of extravascular water protons in a voxel, and is therefore directly related to CBV. There is shown in FIGS. 1A-1C the time course of the inversion-recovery sequence and signals from blood and tissue after an inversion pulse, which illustrate that the longitudinal magnetization relaxes back towards equilibrium after application of this pulse. In the particular application of the technique, a non-selective inversion pulse, i.e. without slice-selection gradient, is used to invert the magnetization so as to minimize or avoid signal dependence on flow effects.
The above-described inversion-recovery sequence technique, however, is limited in that as a practical matter the approach is limited to a single slice or multi-shot 3-D acquisition techniques with limited readout time due to the fact that there is only one zero-crossing point on the T1 relaxation curve as is illustrated in FIG. 1B. Thus, for application of VASO in fMRI, which requires the repeated fast acquisitions (using single-shot techniques) of a portion of the brain (or the entire brain), there are no sequences existing to make it useable in more than a single-slice.
The second example concerns black-blood angiography imaging, and is related to VASO by the fact that the blood needs to be nulled. However, an additional effect can be used in this case that makes it relatively easier to use is the outflow effect. Effectively, the blood needs to be nulled in the vessels only, while maximizing the surrounding tissue signal. For this reason, double-inversion pulses are usually used as preparation schemes for this sequence, in which the first one is non-selective, and the second is slice-selective, in the slice of interest. The magnetization of the inflowing blood will then again relax back, and cross zero at the time of acquisition of this particular slice, while the tissue signal will remain maximal during the whole process, having experienced two 180° pulses resulting in a net 0° pulse. The problem is more complex in multi-slice black-blood imaging, for which very complicated schemes of interleaved inversion pluses have been proposed for only limited volume coverage. These schemes have been used in combination with long echo times to make use of the “outflow” effect, using such sequences as fast spin echo. The problem is then that the available signal in the slice decays with the transverse relaxation time (T2), which is one order of magnitude lower than the longitudinal relaxation times (T1).
Another example relates to the use of preparation sequences in brain perfusion imaging to eliminate the signal from the tissues (gray matter, white matter, CSF), while maximizing the signal from the blood perfusing these tissues. The situation is then the inverse from the two previous example, in which the blood signal was nulled. In order to null the signal of more than one tissue species, more than one inversion pulse is needed. Generally, for N tissues to null, we need N inversion pulses. Several methods using similar principles have been published. Then again, the use of these methods for fMRI or for any application in which rapid imaging is required is restrained by the single time point on the multiple inversion curves of all these tissues at which they cross the zero line. So far, either multi-shot approaches have been proposed, or deviations from the pure zero-crossing point have been used.
Furthermore, in a large number of MR clinical imaging sequences besides the three examples above, it is desired to suppress the large signal coming from lipids, which can produce artifacts, especially in fast sequences, such as spiral or echo-planar imaging. In these other techniques, suppression of the signals from fat uses generally spectrally selective saturation or inversion pulses on the fat signal, or spectral-selective water excitation. The weakness of such methods, however, is that they rely on a very good homogeneity of the main magnetic field Bo, which can be hard to achieve at high fields or in the part of the body placed away from the isocenter of the magnet (e.g., breast imaging), or in areas with strong susceptibility differences (e.g., in cartilage imaging).
It thus would be desirable to provide new MR imaging methods that allow the acquisition of MR images in multiple slices while keeping the signal from the one or more species around the zero-crossing point (i.e., nulled) using non-selective inversion pulses that are aimed towards the continuous nulling of one or more T1 species during the acquisition of multiple slices or sections of a three-dimensional dataset. More particularly, such methods would include interleaving one or more excitation pulses for acquiring MRI image data with non-selective inversion pulses. It would be particularly desirable to provide such methods that would allow acquisition of MR images of successively acquired slices without essentially any contamination by the targeted nulled species. It would be particularly desirable to provide such methods in which the plurality or more of successively acquired slices or sections of a three dimensional dataset are acquired between two sets of one or more signals or pluses being generated to null the one or more species. It also would be desirable to provide systems, devices and apparatuses embodying such methods.